In the field of shearing machines, the state of the art includes cropping machines used to make frames for windows and doors, paintings or furniture; these machines are used to cut to size the elements which make up the frame, starting from a rod or continuous strip made of wood or similar material.
These cropping machines comprise a supporting structure with a horizontal work plane on which the rods which are to be cut are placed and fed forwards.
A shearing assembly is associated with the supporting structure and comprises a movable trolley supporting the blade or blades, or comprises as many movable trolleys as there are blades to move.
The blades can assume at least a working position, wherein they are taken into contact with the work plane where the rods are placed, in order for shearing to be carried out, and an inactive position wherein they are distanced from the work plane and are repositioned for the start of the next cycle. The blades have a shearing position lying on a plane substantially orthogonal to the work plane and angled at 45.degree. with respect to the axis of feed of the rod to be sheared, so that they make a sloping cut on the rod; they are also arranged at an angle of 90.degree. with respect to each other, so as to make two simultaneous and specular cuts of 45.degree..
The shearing assembly can travel in a pendular manner, if the shearing assembly follows an arched trajectory with a centre of rotation outside the work plane, or in a linear manner if it follows a rectilinear trajectory substantially orthogonal to the work plane.
The problem with state of the art cropping machines is that both in the ascent step and in the descent step the blades follow the same trajectory. This means that the sides of the blades, as they ascend, rub against the cut edges of the pieces resting on the work plane and clamped upstream and downstream of the shearing assembly.
This rubbing action can be even more harmful if there is even a little displacement of the two elements of the rod separated by the cutting action due to the release of tensions in the rod as it is cut. The rubbing damages the pieces, both in terms of lesions on the cut edge, and also in terms of scratches on the paint or the decorations.
The continuous rubbing of the sides of the blades may also cause them to wear prematurely, and the quality of the cut to deteriorate, so that the blades need to be replaced very frequently.
This same problem occurs substantially in every type of shearing machine comparable to the one described above, for example in shearing machines where the shearing assembly is movable horizontally, such as those used to shear to size metallic foils or sheets or metallic alloys.
The present applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages.